This is commonly used in roleplay (like D&D, for example) when you're RPing as yourself and must stop to think "What would I do?" before you make your character take an action. Otherwise, you'd say it when faced with a very tough decision. This is somewhat similar to WWJD.
You also say this before someone's going to get themselves killed, and you don't know what you'd do without them.
"I could buy some pizza, or some tacos. WWID?"
2. "WWID if you died? I don't know - that's what!"